1. Field of the Invention
The present invention relates to a process for producing a thin film by sputtering at a high speed and a thin film produced thereby, especially, an optical thin film such as an antireflection coating film. More particularly, the present invention is concerned with a process in which a surface of a film source material is heated and the heated surface is sputtered by ions to thereby produce a thin film and a thin film produced thereby. Further, the present invention relates to an optical instrument including such a thin film.
2. Discussion of Related Art
The vacuum evaporation process has been widely employed in the formation of thin films, especially, optical thin films such as an antireflection coating film, a half mirror, or an edge filter because not only is the processing easy but also the deposition for film formation can be conducted at a high rate.
In recent years, the demand for coating by sputtering in the formation of an optical thin film and other thin films is increasing because of its advantages over the vacuum evaporation process in terms of ease of automation, energy saving, and applicability to substrates with large surface areas.
However, the sputtering process has a drawback in that the film formation is slower than in the vacuum evaporation process In the formation of a metal coating film, the sputtering process is still practicable. However, in the formation of other coating films, the extreme slowness of film formation has tended to delay the industrial spread of the sputtering process. Moreover, the sputtering process has encountered the problem that, in the sputtering of a fluoride such as MgF.sub.2 of low refractive index which provides a typical optical thin film, dissociation into F and species such as Mg occurs so that F is deficient in the coating film to thereby cause the coating film to suffer from absorption of visible radiation.
The above drawback and problem have been a serious obstacle in the application of the sputtering process to formation of optical thin films.
For example, an invention in which the sputtering process has been applied to optical thin films is disclosed in Japanese Patent Application Laid-Open Specification No. 223401/1992.
In the above specification, it is disclosed that, although sputtering of MgF.sub.2 per se leads to absorption of visible radiation, sputtering of a target composed of MgF.sub.2 doped with Si enables formation of a coating film of low refractive index substantially free of light absorption.
In the invention of Japanese Patent Application Laid-Open Specification No. 223401/1992, however, the highest rate of deposition for film formation is only 10 nm/min or less even when a radio frequency power of 500 W (2.8 W/cm.sup.2) is supplied to a 6-inch target. That is, the invention has not overcome the slow film formation drawback of the sputtering process.
When the deposition rate is only 10 nm/min or less, for example, formation of a monolayer antireflection coating film used in the visible region would take 10 min or more. Thus, it is clear why the industrial spread of the disclosed sputtering process is difficult.
Further, follow-up experiments made by the instant applicant in accordance with this prior art have demonstrated that, when a MgF.sub.2 plate having an Si wafer disposed thereon is used as a target and is sputtered, light absorption in the visible region is practically negligible but a coating film with a refractive index of 1.4 or less cannot be formed.